1. Field of the Invention
The present invention relates to an absolute capacitive micro pressure sensor. More particularly, the invention relates to an absolute capacitive micro pressure sensor or transducer subject to exposure to a fluid such as saline when implanted in the body.
2. Description of Related Art
Monocrystalline silicon absolute capacitive micro pressure sensors comprise a relatively thin silicon membrane that mechanically deflects under pressure. The mechanical deflection is correlated to the pressure to be measured. In order to measure an absolute pressure, the other side of the membrane is exposed to a substantially constant pressure created by a sealed chamber.
Capacitive pressure sensors measure the mechanical deflection by measuring the change of the capacitance constituted by a dielectric region between a pair of electrodes or conductive layers, one being mechanically fixed and the other being deflectable or moveable. FIG. 3 is a graphical representation of capacitance as a function of pressure and thickness of the deflectable membrane of a conventional monocrystalline silicon absolute capacitive pressure sensor. Three exemplary thicknesses of the deflectable membrane (45.8 um, 44.3 um, 42.3 um) are depicted by the dotted, dashed and solid lines, respectively. As is evident from the three curves, the thicker the membrane the less deflection of the membrane for :a given pressure and hence the smaller the variation in capacitance.
FIGS. 1A & 1B depict cross-sectional views of two different prior art absolute capacitive micro pressure sensors or transducers including a pressure sensor element 150 with a semiconductor base element 101 (e.g., silicon) mounted, preferably anodically bonded, at interface 110 to a base plate 100, preferably glass, having a hole or opening 112 defined therethrough. Opening 112 has a diameter in a preferred range between approximately 0.5 mm and approximately 2.5 mm.
Sensor element 150 includes a pair of conductive layers or capacitor electrodes separated from one another by a predetermined distance D therebetween. In particular, the pair of conductive layers or electrodes comprises a first mechanically fixed capacitor electrode 106 mounted to a glass element 102 and a deflectable membrane 111 comprising part of the base element 101 so as to be separated from the electrode 106 by a predetermined distance. In the embodiment shown in FIG. 1A, the deflectable membrane 111 is made from a deflectable conductive material such as doped p+ silicon thereby serving as a second moveable capacitor or conductive layer. In an alternative embodiment shown in FIG. 1B, the deflectable membrane 111 is made from a non-conductive semiconductor material such as silicon and a separate second moveable capacitor electrode 113, for example, a conductive metallic layer, is deposited onto the deflectable membrane 111.
The pressure sensors depicted in FIGS. 1A & 1B both have one or more metalized feed through passages 105, 107 defined through the glass element 102. Despite only two being depicted in each of the figures, more than two feed through passages is contemplated and within the intended scope of the present invention. Each feed through passage is bounded on both sides of the glass element 102 by a metalized area. In particular, metalized feed through passage 105 is bounded on one side of the glass element 102 by fixed capacitor electrode 106 and on its opposing side by metalized area 104, while metalized feed through passage 107 is bounded between metalized areas 108, 109.
Fixed capacitor electrode 106, represented in FIGS. 1A & 1B, is positioned so as to be subject to a substantially constant sensing element reference pressure in a sealed cavity 103, while the deflectable membrane 111 is exposed to a pressure to be measured in open cavity 114. In FIGS. 1A & 1B, reference element Z represents the distance between the deflectable membrane 111 and the bonding interface 110 of the base plate 100, whereas H denotes the thickness of base plate 100 itself.
In operation, as the pressure being measured increases the membrane 111 deflects towards the fixed capacitor electrode 106 thereby reducing the distance D therebetween. The capacitance value of a capacitor formed by the fixed capacitor electrode 106 and the membrane 111 as a result of the reduced distance D is sensed to detect the pressure being measured. On the other hand, as the pressure being measured decreases the membrane 111 deflects away from the fixed capacitor electrode 106 causing the distance D therebetween to increase. As a result of the increased distance D, the capacitance value of the capacitor formed by the two electrodes is sensed to determine the pressure being measured. Similar operation occurs with the alternative embodiment shown in FIG. 1B, the only difference being that the moveable capacitor electrode 113, rather than membrane 111, represents the second capacitor electrode.
The conventional absolute capacitive micro pressure sensor configurations shown in FIGS. 1A & 1B are subject to several limitations or drawbacks. Electronic circuitry such as an integrated circuit is used to measure the capacitance of the pressure sensor or transducer representing a combination of two capacitances, that is, the capacitance to be measured CM (variable term) plus a stray capacitance CS (substantially constant term). Stray capacitance is an undesirable, but always present, capacitance that depends on the dielectric of the particular medium to which the deflectable membrane 111 is exposed in open cavity 114. The stray capacitance is produced between the pressure sensor element 150 and electronic circuitry 203 measuring the capacitance of the pressure transducer, as illustrated in FIG. 1C. The dielectric properties of the external medium or environment 213 impact the stray capacitance. The value of the stray capacitance is substantially constant for a particular medium so long as its dielectric properties remain substantially unchanged; however, the value of the substantially constant stray capacitance varies depending on the medium. Preferably, the stray capacitance contribution is calibrated at the time of manufacture. Dry sensor calibration (e.g., when the open cavity 114 is exposed to air) at the time of manufacture is easier to implement and thus preferred over wet sensor calibration (e.g., when the open cavity 114 is exposed to a fluid). Thus, it is desirable to perform a dry calibration of the pressure sensor at the time of manufacture even though the sensor is subsequently used in a different medium (e.g., saline in the case of the sensor being implanted in the body). To achieve this goal, the stray capacitance for the two different medium (e.g., air and saline) is reduced to be negligible, if not zero, by applying a gel in the open cavity in contact with an exposed surface of the deflectable membrane.
Another disadvantage associated with stray capacitance is the deleterious impact on overall sensitivity of the pressure sensor readings or measurements. As mentioned in the preceding paragraph, the capacitance measured by the pressure sensor electronic circuitry represents the combination of CM and CS. The greater the CS, the lower the sensitivity of the CM. By way of a first illustrative example, in an ideal scenario CS=0. If the CM ranges from approximately 5 pF to approximately 15 pF over the pressure range, then a capacitance change of approximately 1 pF corresponds to approximately 6.7% of the maximum capacitance the electronics has to measure (maximum CM of approximately 15 pF+CS (0)=approximately 15 pF). In a second example, CS is approximately 10 pF, while the CM still ranges from approximately 5 pF to approximately 15 pF over the pressure range. A capacitance change of approximately 1 pF corresponds to approximately 4% of the maximum capacitance the electronics has to measure (maximum CM of approximately 15 pF+CS (10 pF)=approximately 25 pF). For greater CS values, the measurement sensitivity will be reduced further still. As a result, minimizing the stray capacitance CS optimizes the capacitance measurement sensitivity.
Yet another disadvantage arises from the deflectable membrane 111 of the absolute capacitive micro pressure sensor being in direct contact with a fluid over a prolonged period of time possibly altering its mechanical properties. Exposure of the membrane 111 to a fluid may result in etching thereby reducing the thickness of the membrane material. Such factors as the salinity and/or pH of the fluid effect the extent of etching. A reduction in the overall thickness of the deflectable membrane 111 will decrease the mechanical stiffness resulting in sensor drift. Growing or depositing of a film or layer (e.g., a protein layer) on the membrane 111 is also possible particularly when exposed to a fluid over an extended period of time. Etching of and/or growing a layer on the deflectable membrane 111 will undesirably alter its mechanical properties and thus result in sensor drift.
Still an additional limitation or drawback of the absolute capacitive micro pressure sensor configuration in FIGS. 1A & 1B is the reduced accuracy of the pressure measurements as a result of formation of air bubbles in the fluid. During manufacture, an opening 112 is drilled in the base plate 100. Sensor element 150 is substantially aligned with the opening 112 and bonded to the base plate 100. Due to the very small size of the opening 112 (typically in the range of approximately 0.5 mm-approximately 2.5 mm) in the base plate 100, an air bubble may form in the cavity 114 as the fluid enters through the opening 112 defined in the base plate 100. The size constraints of the opening 112 are such that the air bubble cannot easily evacuate from the open cavity 114. Changes in fluid pressure within the open cavity 114, in turn, cause the air bubble to compress/expand thereby damping the sensed pressure.
Initially after manufacture, the implantable pressure sensor is not exposed to any fluids. As soon as the sensor is exposed to a fluid (i.e., during fluidic priming prior to implantation or after implantation when exposed to bodily fluids), an air bubble may form as the fluid enters the open cavity 114. Over time (e.g., several days or weeks), the air bubble is slowly absorbed by the fluid. In the meantime, the presence of the air bubble damps the pressure signal.
It is therefore desirable to develop an improved absolute capacitive micro pressure sensor that eliminates or minimizes the aforementioned drawbacks associated with use of an absolute capacitive micro pressure sensor in an environment subject to a fluid such as when the sensor is implanted in the body.